702 A TREATISE ON METAMORPHISM. 



individuals with undiminished transverse dimensions, but with various frac- 

 tions of the average length of those of the background which have not been 

 interfered with by the porphyritic minerals. In short, the appearance, so 

 far as the minerals of the background are concerned, is precisely that which 

 would be produced if one could cut out from a fully developed schist an 

 outline for a porphyritic mineral and subsequently insert that mineral with 

 extreme nicety. (PI. Ill, -D.) 



The minerals included in the porphyritic individuals have the same 

 appearance, and have their granules arranged according to the same sys- 

 tem, as the like minerals in the background of the schist. 



Where in the schists the porphyritic constituents have crystal habits, 

 and are not arranged with their greater dimensions parallel with the schis- 

 tosity of the groundmass, the evidence seems conclusive that such 

 porphyritic crystals developed under mass-static conditions. The case 

 seems especially conclusive when readily cleavable porphyritic minerals, 

 such as mica, chlorite, and chloritoid, in well-defined crystals, occur with 

 their cleavages diagonal or perpendicular to the dimeusional arrangement 

 and cleavage of similar minerals ,of the background. Such cases are 

 beautifully illustrated by the Hudson schist of New York and by the 

 Michigamme schist of the Upper Huronian of the Lake Superior region. 

 Such porphyritic constituents with random orientation formed after the 

 movement ceased during which the minerals with parallel orientation were 

 formed and the schistose structure developed. 



The conditions favorable to the development of porphyritic crystals 

 are those already mentioned as favorable to recrystallization under mass- 

 static conditions, viz, the presence of water, high pressure, and high temper- 

 ature. Where those conditions obtain to an exceptional degree porphyritic 

 constituents are common. These conditions are likely to obtain to an 

 unusual deg-ree after either powerful orogenic movements or great batho- 

 lithic intrusions, or, still more commonly, the two combined. This is 

 illustrated by the porphyritic crystals of nearly every variety, including 

 mica, chloritoid, andalusite, garnet, staurolite, and tourmaline, in the schists 

 adjacent to the great Black Hills a granite batholith, and by the many 



« Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 

 1890, pp. 222-230. 



